In the fabrication of semiconductor devices, several layers of metallization are built on top of each other separated by insulating layers- The metallization typically is formed by the deposition of aluminum. Aluminum is used rather than copper to form leads and other metallization structures on a semiconductor device despite the fact that copper has lower resistance than aluminum due to problems associated with the deposition and etching of copper. Specifically, when copper is etched, it does not form a volatile gas with the plasma to allow removal from the processing chamber. Once copper is etched, it tends to be redeposited either elsewhere on the semiconductor device or on the processing chamber itself. In either case, contamination by copper in unwanted locations can destroy the circuits being constructed.
In an attempt to obtain the benefits of copper and avoid the problems with copper etching, the use of a thin sputtered seed layer of copper followed by a thick plated conductor layer has been developed- The copper seed layer method comprises depositing over a plasma oxide a multi-layered structure comprising a conductive barrier layer, a seed layer and a third protective layer. A standard photoresist process forms patterns within the photoresist over the multilayered structure. Once the patterns have been formed within the photoresist, the third protective layer is removed by reactive ion etching with carbon tetrachloride. Once the third protective layer has been removed, copper may be electroplated onto the seed layer, followed by a solvent strip to remove the resist.
The electroplated copper device thus formed has negatively sloped sidewalls due to inaccuracies inherent with the photolithographic process. Nonvertical sidewalls may result in inconsistencies in further processing, which may damage the functioning of the device being formed. Additionally, a step of etching is required to remove that portion of the seed layer not covered by the electroplated copper and to isolate the plated leads. As previously stated above, etching copper allows the unwanted redeposition of the etched material.
Another method known as "lift-off" has been developed for metallization processing but has only been used with aluminum. In addition to the disadvantages previously stated above, aluminum does riot provide a structure with sufficiently fine lines or small enough pitch for the increasingly down-scaled devices that are currently being designed. Aluminum must also be deposited in relatively thick layers which can add to the overall dimensions of the device. Thus, there is a need for a method for forming a copper metallization device on integrated circuits that has vertical sidewalls and does not require the etching of copper.